1. Field of the Invention
The present invention relates to a drive mechanism which employs an electromechanical transducer, a photographing lens having the drive mechanism, and a drive circuit for driving the drive mechanism.
2. Description of the Related Arts
Conventionally, there have been proposed a variety of actuators, each of which employs a piezoelectric element as an electromechanical transducer. For example, the piezoelectric element of an actuator, which constitutes a part of a photographing lens, of a camera in which the actuator is employed for executing a focusing operation (i.e. for driving a focus lens) of the photographing lens, is supplied with a drive pulse with a frequency that is higher than a frequency that corresponds to an upper limit of audibility. With the supply of such a drive pulse to the piezoelectric element, a reduction in mechanical noise which is generated from the actuator upon executing the focusing operation, is realized.
When the focusing operation is executed under a focusing drive control, a focus lens, which is included in the photographing lens, is not driven at a constant speed or velocity.
For example, if the focusing drive control is an "AF" control for auto-focusing, the speed of the focus lens is reduced as the focus lens approaches a desired target location, so that the focus lens is prevented from passing beyond the desired target location.
On the other, hand if the focusing drive control is a "PF" control for manual-focusing which is executed in the case that the photographing lens is a power focusing lens (namely, "PF" lens), the focus lens is driven by the actuator at a speed, or velocity, in accordance with an amount of operation (or operation speed) of a focus ring, of the photographing lens, which is operated by a photographer. The "PF" control is a power focus control which generally means the focus control to control a manual focus lens-which is driven by a driving source, such as a motor, for performing its focusing operation.
Generally, a maximum drive speed, or velocity, of the focus lens, depends on capability, or performance, of the actuator. In order to drive the focus lens at a desired speed or velocity, it has been conventionally practiced to execute a feedback control for adjusting a drive voltage which is supplied to the actuator. Namely, the drive voltage is being adjusted while an amount of movement, or displacement, of the focus lens is being monitored.
The feedback control, however, has a problem as follows. Namely, when the focus lens approaches the desired target location under the "AF" control, there is a possibility that a low-velocity driving of the focus lens is not realized.
On the other hand, the operation speed of the focus ring is small under the "PF" control, and there is the same possibility that a low-velocity driving of the focus lens is not realized.
This problem is caused by a variation in characteristic of speed, or velocity, of the actuator for driving the focus lens, and is caused by a limit of controllability of the feedback system.
More specifically, for example, as shown in FIG. 1 which denotes a characteristic between a drive voltage of the actuator and the speed, or velocity, thereof, the characteristic therebetween varies depending upon, for example, an error in assemblage of the individual actuator. In FIG. 1, a curve "a" is generally linear in relation between the drive voltage and the velocity.
However, a curve a "b" and a curve "c" are generally non-linear in relation therebetween, respectively. Namely, each of the curve "b" and "c" indicates that the actuator is not driven until the drive voltage is increased up to a certain level of voltage.
Furthermore, each of the curve "b " and "c" indicates that the actuator suddenly, or abruptly, speeds up once it starts moving.
As can be seen from FIG. 1, the performance, or characteristic, of the individual actuator varies on a basis of its individual peculiarity thereof, even if the same drive voltage is applied to it. In particular, in the case that an inclination of a curve, as illustrated by the curve "c", is steeper at a low velocity, the speed of the actuator changes more greatly even if the drive voltage applied thereto changes a little. This means that it is very difficult to smoothly control the drive, or operation, of the actuator by the feedback control at the time of driving the actuator at such a lower velocity.
FIGS. 2A and 2B show an example of controlling the actuators having characteristics denoted by the curves "a", "b" and "c" of FIG. 1. In this example, a drive pulse is continuously supplied to the actuator so as to control the drive of the actuator. Namely, as shown in the figures, the amplitude of the drive voltage supplied thereto is reduced step by step, so that the velocity of the actuator is gradually reduced for the purpose of preventing the focus lens, driven by the actuator, from overrunning its target location.
In case that the actuators having characteristics denoted by the curves "a", "b" and "c" of FIG. 1 are controlled in the same way with the same drive voltage shown in. FIG. 2A, each of the actuators having characteristics denoted by the curves "b" and "c" stops before it reaches its target location as the amplitude of the drive voltage becomes smaller, as shown in FIG. 2B.
Meanwhile, FIGS. 3A and 3B show an example of controlling the actuator, having the characteristic denoted by the curve "c" of FIG. 1, under the conventional "AF" control. In this example, the control of velocity of the focus lens is executed only by the feedback of the drive voltage supplied thereto, as shown in the figures. As explained above, the velocity of the actuator having the characteristic denoted by the curve "c" of FIG. 1 changes greatly, in case that the drive voltage is at a lower level, and in case that there is a slight change in the drive voltage. Therefore, in case that the drive voltage is supplied to the actuator, under the feedback control, so as to realize a lower required velocity (see "REQUIRED VELOCITY") denoted by a chain line in FIG. 3B, the actuator repeatedly moves and stops as shown by a solid line in the same figure, depending upon a fluctuation of the drive voltage under the feedback control. In other words, the movement of the actuator lacks smoothness; therefore, such an awkward movement of the actuator may give a user (i.e. a photographer) a feeling of unpleasantness.
That is, as explained above, in case that there exists a variation in characteristic between the drive voltage applied to the actuator and the velocity thereof (namely, in case that the characteristic is not generally linear therebetween), it may not possible to realize a desired target velocity of the actuator and/or the actuator may suddenly stops even if controlling the drive voltage by executing the feedback operation.
In order to control the actuators having such characteristics with a higher accuracy at a lower speed, it is necessary to increase its positional detection accuracy and/or to shorten its sampling cycle of the positional detection. For example, if the positional detection accuracy is 1 .mu.m, and if the sampling cycle of the positional detection is 1 ms, then its controllable limit velocity is 1 .mu.m/1 ms=1mm/s. If a required velocity is 0.1 mm/s, then it is necessary to further increase the positional detection accuracy and/or to further shorten the sampling cycle of the positional detection.
However, it is very difficult to further increase the positional detection accuracy, because its countermeasure to realize the higher positional detection accuracy is accompanied with some technical problem. Also, it is very difficult to further shorten the sampling cycle of the positional detection, because the processor, which executes an operation to detect the position, can not help but require some time for the operation.
In order to realize the drive of the actuator at the lower velocity, it is possible to use such a simplest mechanism as employing a drive pulse having a lower frequency. However, if the frequency is lowered up to a zone, or range, of frequency which corresponds to audibility, a noise of mechanical vibration caused by the drive pulse is emitted. This noise spoils a feature, which is a silent working and operation, of this actuator.